Assessing the strengths of solar thermal desalination

(Nanowerk News) The technology behind solar thermal desalination (STD) - ridding salt from water with help from the sun - has been around for centuries. In recent years, though, it has gained a fair amount of attention from researchers aiming to improve its performance as a desalination technology. Much of that attention has focused on developing novel materials that better absorb light, as well as system design.

Researchers at Yale and Vanderbilt have collaborated to propose an elegant theoretical framework to help others in the field better understand how various technological innovations can enhance STD performance. The framework is summed up in a straightforward equation that can be universally applied to all STD systems, including those that have not yet been developed.

"With this framework, we can now assess the significance of a specific technological improvement in the bigger picture of enhancing STD efficiency," said Zhangxin Wang, lead author of the paper, and a postdoctoral fellow in the laboratory of Menachem Elimelech, the Roberto C. Goizueta Professor of Chemical & Environmental Engineering.

"We're in the stage of this technology where people are still trying to improve the efficiency of vapor generation," Wang said. However, he noted, the framework clearly shows that this attention may be better directed elsewhere. Specifically, improving the recovery of latent heat of condensation – that is, the heat released when vapor cools down to become liquid water - is a much more promising way to enhance STD performance than further perfecting the efficiency of vapor generation.

"The framework also helps us evaluate the limitations of STD," said Shihong Lin, the co-corresponding author and assistant professor of civil and environmental engineering at Vanderbilt University. "It suggests that it’s highly unlikely that, no matter how we improve STD, the efficiency of this technology will approach that of the more conventional ways of solar desalination, such as coupling photovoltaic panels with reverse osmosis (PV-RO) " he added.

Another thing to consider is that STD technology requires a much bigger area. The high efficiency of PV-RO means that it doesn't take up a lot of space - perfect for places like California where land use is a big concern. "Footprint is a critical consideration for any municipal-scale water treatment facility and STD is unlikely to be competitive in that market," Lin said.

When it comes to STD technology, the researchers say, the real potential benefit is its low cost. PV-RO requires pricey components such as solar panels, high-pressure pumps, and membranes.

"It's all very expensive compared to the cheaper materials that can be used for STD," Wang said. "These STD systems may use 50 times more area, but still end up being cheaper than PV-ROs. You can build these with inexpensive natural materials and carbonize them to make them really effective for solar-driven evaporation."

Considering the costs, STD technology is most suited for niche applications.

“For instance, it could be used in in remote areas, or developing countries with no infrastructure for energy and where the capital resources are limited, but land use is not a concern," Lin said.